1. Technical Field
The present invention relates to methods for driving ferroelectric memory devices, and more particularly, to methods for driving ferroelectric memory devices that store data set with readout restriction.
2. Related Art
A ferroelectric memory device (FeRAM: Ferroelectric Random Access Memory) is a memory device that uses the property (remnant polarization property) of a ferroelectric film in which the film is polarized by an electric filed applied externally, and the polarization remains even when the external electric field is removed. By changing the direction of electric field, the direction of polarization of ferroelectrics changes, whereby data can be rewritten.
On the other hand, the number of readout operations of a memory device may be restricted in view of management of data to be stored. For example, depending on the confidentiality of stored data, the data may require to be managed with such a limitation that allows data readout only once.
For example, Japanese Laid-open Patent Application JP-A-2003-91988 (Patent Document 1) describes a system that uses a ferroelectric memory device whose data stored in a memory can be automatically erased after the data has been read out once.
A memory device equipped with the function to erase information immediately after the information has been readout, as described in Patent Document 1, may be compared to an attaché case for carrying confidential documents. When information stored in the memory device is highly confidential, a state in which the information remains within the memory device even after the information has been readout corresponds to a state in which, even after confidential document has been removed, a copy of the confidential document remains in the attaché case. This increases the danger of information leakage, and may lead to a failure of the control of information. In this respect, in the system described in Patent Document 1, the memory device is equipped with the function to “have confidential document in the attaché case non-existing after the confidential document has been removed” such that the memory device is provided with a role of an attaché case for safely carrying confidential document.
However, information stored in a ferroelectric memory device can be readily rewritten because of its own characteristic as a memory device. For example, referring to the example described above, this may be compared to a situation in which, when new confidential document is placed in the attaché case, the confidential document that has been in the attaché case is erased.
Therefore, methods that are capable of managing confidential information while preventing elimination of the confidential information are desired to be examined.